1. Field of the Invention
The present invention relates to a field-effect transistor (referred to as a FET hereinafter) having an ohmic electrode, and in particular to a semiconductor device having a photosensitive organic film causing its gate parasitic capacitance to be reduced and a method for producing the same.
2. Related Art
As semiconductor devices are made minute in recent years, electric signals propagating in wires of the semiconductor devices delay by the resistance of the wires or the capacitance between the wires. As the wire resistance or the capacitance between the wires is smaller, the delay of the electric signals becomes smaller. For this reason, it is important to make the gate parasitic capacitance small.
FIGS. 1A to 1E illustrate steps of producing the conventional semiconductor device having an ohmic electrode in the order of the steps. As shown in FIG. 1E, in a conventional semiconductor device 100 a recess 104 is made in a semiconductor substrate 101, and a gate electrode 106 in the form of "T" along a vertically sectional direction of the semiconductor substrate 101 is fitted to the recess 104. A protective film 107 having a uniform thickness is deposited on the semiconductor substrate 101 and the peripheral face of the gate electrode 106. Holes 109 are made at both sides of the gate electrode 106 and in parts of the protective film 107. Respective ohmic electrodes 110 are formed in the respective holes 109 in the manner that the electrodes 110 contact the semiconductor substrate 100 directly.
As illustrated in FIG. 1A, an oxide film of, for example, 4000 .ANG. in thickness, as an insulating film 102, is first deposited on the semiconductor substrate 101. Thereafter, the insulating film 102 is subjected to dry-etching through a photoresist as a mask to make an opening 103 having an aperture size of, for example, 0.5 .mu.m. Subsequently, the semiconductor substrate 101 is etched off by a thickness of, for example, 1000 .ANG. to make the recess 104.
Next, as illustrated in FIG. 1B, a side wall oxide film 105 of, for example, 3000 .ANG. in thickness is deposited. By such a side wall working, the aperture size of the opening 103 is reduced up to, for example, 0.2 .mu.m. For example, WSi, as a gate metal, is then sputtered thereto to form the gate electrode 106.
Next, as illustrated in FIG. 1C, the insulating film 102 and the side oxide film 105 are wholly removed to reduce gate parasitic capacitance. Thereafter, a thin oxide or nitride film of, for example, 1000 .ANG. in thickness is again deposited as the protective film 107.
Next, as illustrated in FIG. 1D, the holes 109 are made in the protective film 107, using a photoresist 108 as a mask.
Next, as illustrated in FIG. 1E, for example, AuGe for making the ohmic electrodes 110 is vapor-deposited into the holes 109 made in the protective film 107. The photoresist is lifted off to complete the semiconductor device 100.
In such a conventional semiconductor device, however, the gate parasitic capacitance varies in accordance with the thickness of the deposited oxide film or nitride film. As a result, there remains a problem that the performances of FETs vary largely.
It is general according to the above mentioned conventional producing process that after the gate electrode is formed, the oxide film formed under a head overhanging portion of the gate electrode is removed and then the thin oxide film or nitride film is deposited as the protective film. Since the oxide or nitride film remains as the protective film under the head overhanging portion of the gate electrode, there also arises a problem that the gate parasitic capacitance cannot be sufficiently reduced.
Furthermore, in the case that the oxide film on the portions where the ohmic electrodes should be formed is removed, the oxide film inside the recess is also etched by over-etching. In this way, a portion of the semiconductor substrate between the gate electrode and the recess is taken away. As a result, there is caused a problem that the performances of FETs vary.